Apparatus for controlling glass fractures

ABSTRACT

This invention relates to apparatus for arresting a developing fracture in a ribbon of glass by the application of heat of about 950 degrees F. to the area of the fracture and inducing temporary thermal stresses which place the glass in compression, thus arresting and eventually controlling the fracture. The apparatus comprises a crack-sensing device, a control means and a high-intensity heat source which are mounted so as to scan continuously the moving ribbon, and with the heat source being activated in response to the presence of a developing crack. Circuits may be used to move the heat source across the width od the glass ribbon to direct the fracture toward an edge of the ribbon.

Sept. 26, 1972 T. A. DEAR 3,594,131

APPARATUS FOR CONTROLLING GLASS FRACTURES Original Filed Nov. 29, 1968 INVENTOR T RREME A. 0.54%

BY OMMW Wm ATTORNFY5 United States Patent 3,694,181 APPARATUS FOR CONTROLLING GLASS FRACTURES Terrence A. Dear, Allison Park, Pa., assignor to PPG Industries, Inc., Pittsburgh, Pa.

Continuationof abandoned application Ser. No. 779,730, Nov. 29, 1968. This applicationxJune 1, 1971, Ser.

Int. Cl. C03b 29/00 ABSTRACT on THE DISCLOSURE This invention relates to apparatus for arresting a developing fracture in a ribbon of glass by the application of heat of about 950 F. to the area of the fracture and inducing temporary thermal stresses which place the glass in compression, thus arresting and eventually controll ng the fracture. The apparatus comprises a crack-sensing device, a control means and a high-intensity heat source which are mounted so as to scan continuously the moving ribbon, and with the heat source being activated in response to the presence of a developing crack. Qircuits may be used to move the hea t source across the width of the. glass'ribbon todirect the fracture toward an edge of the ribbon;

This application is a continuation of Ser. No. 779,730, filed Nov. 29, 1968, now abandoned.

SUMMARY OF THE INVENTION In the manufacture of flat glass, ribbons of glass are formed by any one of a number of known processes and conveyed from the forming area through various treating zones and thence to a cutting station where the ribbon is cut into sheets. The ribbon may be conveyed vertically, or first vertically and then horizontally, as in some window-glass drawing operations, or it may be conveyed horizontally, as in the plateand float-glass operatlons. In some plate-glass operations, grinding or both grinding and polishing of the ribbon may be done between the forming zone and the severing station. In all such continuous operations, the ribbon of glass is at a relatively high temperature, around 2000 F. and above, immediately after being formed and is cooled to a relatively low temperature, i.e. around 300 F. or lower, prior to cutting. In most operations the ribbon is gradually cooled in a controlled manner through the annealing range of the glass and thereafter is allowed to cool at a natural rate.

A ribbon of glass at temperatures near the lower limit of the annealing range and below, e.g. about 950 F. and below, is solidified to the extent that there is little or no viscous flow of the glass due to thermal or physical stresses; hence, at these temperatures, the ribbon is capable of cracking or splitting and, in fact, it often does crack or split, particularly because of stresses caused by uneven cooling. When these splits occur down the central portion of the ribbon, parallel to the direction of ribbon travel, they often extend from the end of the ribbon at the severing station back to near the forming zone, i.e., perhaps a distance of over 200 feet. Furthermore, unless steps are taken to heal the split, it may exist for several days, resulting in the loss of all the glass during such period. A known method of healing such splits is to place a heated brick upon the ribbon upstream of the split and in a location where it is estimated the split will occur when that portion of the ribbon supporting the brick reaches the location of where the split is being generated. It is not uncommon to experience an average 3,694,181 Patented Sept. 26, 1972 of about one split per day on a single line, with the length of the splits averaging more than feet. With a standard ribbon width of between 8 and 12 feet, a c onsiderable amount of glass is wasted due to ribbon splits.

Another known method of reducing splits comprises the application of compressive forces against the lateral edges of the moving ribbon. The forces are applied at locations where the ribbon is sufficiently cool to resist substantial deformation by the forces and at points where the ribbon would be likely to split. However, the locations must be determined by observation once a split occurs by observing the location of the generation of a split and then applying forces downstream. This method requires additional equipment, results in some damage to the edges of the glass, and is not fool-proof in its prevention of splits.

This invention provides a method of arresting and controlling fractures by placing the surface of the glass in compression by the application of a concept of effecting an area of high heat transfer immediately in advance of, i.e., upstream of, the running fracture front. The fracture is arrested in its forward direction, and the fracture is guided to an edge of the ribbon by the arresting area of high heat transfer. One of the most appropriate applications of this invention is in a sheet-glass drawing machine, wherein the ribbon is particularly vulnerable to random fractures propagating from the free or severed end toward the drawing-kiln area. A mechanism for fracture detection and a source for the establishment of a high rate of heat transfer, working in conjunction, arrest the fracture and control it .by taking the fracture to an edge of the sheet, thus preventing the so-called rundown of the fracture.

BRIEF DESCRIPTION One form of the invention is shown in the drawing, which may be briefly described as a perspective view of a moving ribbon and assembled associated heat-transfer devices.

With reference to the drawing, there is illustrated a continuous ribbon of glass 10 formed from a molten pool of glass 11 and withdrawn vertically upward through a glassdrawing machine and annealing lehr to a severing station. A heat source 13 is disposed across the ribbon. A fracturedetection apparatus 14 is also immediately disposed above the ribbon. Such suitable heat sources include infrared-spot sources, lasers, flames, plasmas, conduction probes and other heat-transfer mechanisms.

The fracture-detecting device continuously scans the ribbon for the development of a propagating fracture, and when a propagating fracture is detected, the heat source is activated and a zone of the glass is heated to about 950 F. The developing heated zone arrests the propagation of the fracture, and the heat source is then scanned laterally along track 15, directing the propagating fracture to the edge of the ribbon. This action prevents rundown along the ribbon to themolten glass pool.

OPERATION The application of heat in a high quantity to the surface of a glass wherein a crack is propagating immediately begins to place the surfaces of the glass in compression. It is believed that this compressive effect is the mechanism for arresting of the propagating fracture. The application of almost instantaneous heat sources to the surface of the glass causes the induction of temporary thermal stresses in the glass, the result of which is to place the glass sheet in compression. The heat source, being in the range of 2000 to 3000 degrees Kelvin without contacting the surface of the glass, induces the development of the temporary thermal stresses, and without the necessity of bringing anything into physical contact with the glass,

The apparatus of the present invention comprises a w fracture-detection apparatus '14, a source of high thermal energy 13, and an associated control circuit (not shown) for sending a signal to a control means 20 from crack detector 14 when the detector 14 senses a crack. Upon receipt of this signal from detector 14, control means 20 actuates the source of thermal energy 13 so that it applies heat in a high quantity to the surface of the glass wherein the surface of the glass is placed in compression and the crack is propagated. Track 15 steers the movement of detector 14 and thermal energy source 13 across the sheet of glass. A moving ribbon of glass 10 is supported by rolls 12. The ribbon may be either in a horizontal or vertical position, depending upon the manufacturing process used. A support bar 15 provides a rack for mounting the detection means 14 and heat source 13. A crack detector 14, which may be of a type responsive to the variations in intensity of the light or of a type based on a principle of the scattering of light by a developing crack or of the refractive type, responds to the presence or absence of a crack and transmits a suitable impulse to the control means 20. The control means is designed to be responsive to a signal from crack detector 14 and actuates a heat source 13. Any suitable heat source, such as a hot wire, a quartz lamp, a high-intensity gas flame, or a plasma arc may be used. However, electrical sources appear tov be best suited for this type of use, in that they respond almost instantaneously to the presence or absence of the develop ing fracture. As a fracture develops, the detector immediately records it, and the heat source is applied instantaneously. The very quick development of temporary surface stresses places the surface of the glass in compression, which almost immediately arrests the progress of the crack. Other circuits 21 may be used to move the heat source quickly to one or the other side, causing the propagating fracture to run to the side of the ribbon.

Iclaim: Y

1. Apparatus for arresting the progress of a developing fracture in a ribbon of glass, said apparatus comprising, in combination,

means for detecting the presence of said fracture,

control means responsive to said fracture-detecting means, and

means responsive to said control means and separated from said ribbon of glass for heating said ribbon of glass below its melting point to induce temporary thermal stresses which place the glass in compression in a vicinity into which said developing fracture is coming, whereby propagation of said fracture is prevented.

2. Apparatus as defined in claim 1, further including circuit means for moving the heating means across the width of said ribbon of glass to direct said fracture toward an edge of said ribbon.

3. Apparatus as defined in claim 1, characterized in that said apparatus forms a part of a combination with apparatus for the vertical drawing of window glass.

4. Apparatus as defined in claim 3, further including circuit means for moving the heating means across the width of said ribbon of glass to direct said fracture toward an edge of said ribbon.

References Cited UNITED STATES PATENTS 2,197,811 4/1940 Spinasse -Q.-- 28 2,417,094 3/1947 Spinasse l. 65-194 X 3,350,188 10/1967 Barrett et al 65-28 X 3,539,324 11/1970 Terakado ct al 65-28 X ROBERT L. LINDSAY, In, Primary Examiner US. 01. X.R. 

